This project intends to analyze the lethal effects of ionizing radiations and chemotherapeutic drugs on an established human colon carcinoma cell line in order to provide useful information relevant to the rational design of chemo- and radiotherapeutic clinical trials. Cell killing defined as the inability to form colonies, will be studied after exposure to various drug concentrations or radiation doses at different time schedules. Cell cycle stage sensitivity will be investigated by treating synchronized cells. Effects on cell progression delay will be studied aiming to define new synchronizing agents. Cumulative effects of fractionated doses will be defined to estimate cell repair. Therapeutic gain will be investigated by combination of drug treatments simultaneously or sequentially. The response of resting phase populations will be analyzed by the use of plateau phase cultures.